1. Field of the Invention
The present invention is directed to methods and apparatuses which improve phase noise performance of signal generation equipment requiring low phase-noise signal generators. In particular, the present invention is directed towards cancelling the phase noise of an entire system.
2. Description of the Related Art
Phase noise is a major limiting factor in determining, for example, radar dynamic range. The combination of a few ultra-low phase noise, state-of-the-art amplifiers exceeds present and future phase noise requirements for an entire STALO (stable local oscillator). However, every device included in a STALO makes a small addition to the total phase noise performance. When all small additions are combined, the radar dynamic range becomes limited.
FIG. 1a depicts the basic problem to which the present invention is directed. A frequency multiplier 14 of multiple N is driven by a master oscillator 10 of a frequency f.sub.i and of a phase noise spectrum .phi.N.sub.1, indicated at 12. At an output frequency of N.times.f.sub.i, the master oscillator noise spectrum is degraded by N.times..phi.N.sub.1. There are also numerous additive noise sources, whose total is represented by .phi.N.sub.2, indicated at 16. The additive noise .phi.N.sub.2 represents the sum of individual independent noise spectrums of any one of amplifiers, switches, frequency dividers, mixers, resistors and/or diodes. The total output noise spectrum is N.times..phi.N.sub.1 +.phi.N.sub.2.
Currently, attempts to reduce such noise output have focused on reducing noise from the respective sources, e.g., .phi.N.sub.1, components of .phi.N.sub.2 and N itself, but there are definite limits to which these parameters can be reduced.
FIG. 1b illustrates an example of the problem encountered by the configuration shown in FIG. 1a. In FIG. 1b, the plot 18 indicates the phase noise, or Halford noise, and the line 20 indicates the target return. The horizontal axis is the offset frequency and the vertical axis is the signal strength. Such phase noise occurs on an offset frequency range from 0.001 Hz to greater than 10 MHz from the desired signal. The amplitude of the phase noise typically decreases at a rate of 10 decibels per decade of offset frequency.
If such noise is transmitted on a radar signal, or present in the radar receiver local oscillator mixer port, this phase noise can cover up the desired doppler shifted radar return, thus preventing detection of the radar target.
When all of the small additions to phase noise from all of the sources noted above are combined, the radar dynamic range becomes limited, even when using signal generation equipment having improved phase noise. The present state of the art is limited by a summation of the many small contributions.